2: Physiology of the Visual Cortex

Question 1

Describe the laminar organization of the lateral geniculate nucleus.

Answer 1

LGN has 6 layers, 3 from each eye, alternating

Outer 4 layers = PARVOCELLULAR LAYERS: receive info about COLOR vision, fine discrimination of shape

Inner 2 layers = MAGNOCELLULAR LAYERS: receive info about MOTION DETECTION, depth, and contrast

–Most input to LGN (80-90%) is from cortex to regulate input of visual information

Question 2

Diagram the visual projections from the retina to the lateral geniculate nucleus.

Answer 2

Left visual field -> right LGN

Right visual field -> left LGN

Question 3

Describe the visual deficits from lesions in the optic nerve, optic chiasm, optic tract, LGN, optic radiation, and Meyer’s loop.

Answer 3

Damage to optic nerve: monocular blindness

Damage to optic chiasm: lose outer parts of visual fields in both eyes (bitemporal hemianopia

Damage to optic tract, LGN, or optic radiation: lose vision from the opposite side (homonymous hemianopia)

Damage to Meyer’s loop: loss of upper half of visual field

Question 4

Discuss simple cells of the visual cortex in terms of their responsiveness to visual stimuli.

Answer 4

SIMPLE CELLS: receptive fields = combination of information from RETINAL GANGLION CELLS

• -Best stimulus = a BAR of light oriented across the centers of the ganglion cells in the field
• -Might have diagonally, horizontally, or vertically oriented ganglion cells (or somewhere in between)

Question 5

Discuss the role of ocular dominance columns in the organization of the primary visual cortex.

Answer 5

Inputs from both eyes are organized into OCULAR DOMINANCE COLUMNS in the primary visual cortex

• -The cells receiving input from a small piece of retina in one eye (an OCULAR DOMINANCE COLUMN) are next to cells receiving input from that same piece of retina in the other eye
• —Have alternating columns of cells receiving information

Question 6

Discuss the concept of parallel processing of visual information and the role of the several visual association areas in the detection of various aspects of visual perception.

Answer 6

Visual association areas take you from a flat image of the world to COLOR, MOTION, and DEPTH

Projections from V1 (primary visual cortex) to…

• -V2, V3 (occipital lobe): depth perception
• -MT/V5 (middle temporal area): motion detection
• -V4 (occipital lobe): color vision

Question 7

Describe the concept of the ventral and dorsal stream of visual information.

Answer 7

Dorsal pathway: motion, depth perception, relative size
–V1 -> V2, MT

Ventral pathway: object/form recognition (including color)
–V1 -> V2, V4

Question 8

Describe the mechanisms involved in motion detection.

Answer 8

Motion:

1. If you hold your eyes still, does the image move on the retina?
2. Does the image stay still on the retina when you are moving your eyes?

Question 9

How are ocular dominance columns established? (2)

Answer 9

Gradients of Eph receptors and their Ephrin ligands are important for the topographic projection from the LGN to the primary visual cortex

• -Aid in initial localization of axons from each eye into ocular dominance columns
• -Final pruning/localization is from coordinated electrical activity

SUMMARY: BOTH chemical cues (eph/ephrin) and coordinated electrical activity are necessary for the correct segregation of projections and appearance of ocular dominance columns that occurs during development of the primary visual cortex

Question 10

What is strabismus?

Answer 10

Misalignment of the eyes

• -Eyes not aligned in the same direction and do not move together
• -A cause of amblyopia (lazy eye) -> reduced or lost vision from one eye as brain ignores input

Results form lack of correlated electrical activity from each eye

Question 11

What are monocular cues used for? How do they work? (6)

Answer 11

Depth perception - for very long distances (>100-200 feet)

• -Previous familiarity (know how big an adult person is, so if a person is smaller, she is farther away)
• -Relative size
• -Linear perspective: parallel lines converge with distance
• -Interposition: shapes that are interrupted by other ones (man stands behind desk, can’t see legs, must be behind desk)
• -Shadows/illumination
• -Motion parallax: things closer to you move faster, things farther away move slower

Question 12

What are binocular cues used for? How do they work?

Answer 12

Binocular cues (stereopsis): for closer things

• -Have BINOCULAR CELLS with receptive fields on both retinas
• -Maximum response for binocular cortical cell occurs for stimuli falling simultaneously on each eye’s receptive field
• -Binocular cells with receptive fields that are FARTHER away from each other (out towards the periphery of the retina) will signal that an object is CLOSE when they are stimulated
• -Binocular cells with receptive fields that are CLOSER together (near the middle of the retina) will signal that an object is FARTHER away when they are stimulated

Question 13

What characteristics of color do we detect? Define constancy, hue, brightness, saturation, and gradiation.

Answer 13

Color constancy: the property of an object (its color)

Color perception:

• -Hue: proportion to which each cone type is stimulated
• -Brightness: total amount of stimulation of all three cone systems
• -Saturation: how much all three cone systems are stimulated to the same degree

Color gradiation: combination of hues, brightness, and saturation

Question 14

How do we detect color?

Answer 14

Single-opponent color cells: in RETINA; have center-surround organization

• -Red on, green off
• -Green on, red off
• -Blue on, yellow off
• -Red on, blue off

Double opponent color cells: in CORTEX

• -Organized as if response is due to multiple single-opponent color cells
• -Example: best stimulus is red spot in the middle of a green surround

Question 15

What is the binding problem?

Answer 15

Problem: how do we take all our extracted information and create a coherent image?

• -Look for similarities/cues, compare it to things we know
• -From visual field -> feature maps -> binding and merging -> master map
• —Binding and merging probably takes place in the parietal lobe

Question 16

What are aperceptive visual agnosias?

Answer 16

Difficulty separating figures from the background

Wouldn’t be able to recognize a crossed-out figure

Question 17

What is synesthesia?

Answer 17

A cross-modality activation of sensory systems

• -Occurs in 1/10000 people
• -Perhaps a result of excess connections between areas

Examples:

• -Hear saxophone, see neon purple snakes
• -Harmonica sounds green
• -Taste of mango sorbet makes wall look lime green
• -Tuning a violin by what it tastes like (grass if in tune, weeds/dirt if out of tune)

Question 18

Diagram the visual projections from the lateral geniculate to primary visual cortex.

Answer 18

Form the OPTIC RADIATION

–Information from LOWER half of visual field (upper retinal quadrants) -> visual cortex ABOVE calcarine sulcus

–MEYER’S LOOP: information from UPPER half of visual field (inferior retinal quadrants) -> visual cortex BELOW calcarine sulcus

Question 19

Which region of the retina is most heavily represented in the primary visual cortex?

Answer 19

Foveal region has disproportionately large representation in the primary visual cortex

Question 20

In what orientation is the visual image map on the primary visual cortex?

Answer 20

Image in primary visual cortex is upside-down and reversed right to left
–Projected point to point from retina to LGN to visual cortex, but inverted/reversed based on how it hits the retina

Question 21

What molecules aid in signaling for the selective decussation of nerve fibers from the nasal half of each retina at the optic chiasm?

Answer 21

Ephrins!

Temporal retinal ganglion cells express Eph B1, which makes them repulsed by the cells of the optic chiasm, which express Eph B2

Therefore, only axons from the nasal retina cross at the chiasm

Question 22

Describe the laminar and modular organization of the primary visual cortex. Where do inputs from the LGN terminate?

Answer 22

• -Has six layers
• -Includes pyramidal cells and nonpyramidal cells (interneurons)

–All INPUTS terminate in LAYER 4

–Subsequent projections to layers 2/3, then 5, then 6

–OUTPUTS from layers 2-6 (2/3 -> other cortical areas; 6 -> LGN)

Question 23

Discuss hypercomplex cortical cells in terms of their responsiveness to visual stimuli.

Answer 23

HYPERCOMPLEX CELLS: require MOTION of the appropriate stimulus in the correct direction through their receptive field
–Bar of light must be oriented correctly, only stimulating excitatory simple cell, and MOVING

Question 24

Discuss complex cortical cells in terms of their responsiveness to visual stimuli.

Answer 24

COMPLEX CELLS: receptive fields = combination of information from SIMPLE CELLS

• -Sensitive to orientation and length of light stimulus bar
• -Best at detecting the ENDS of lines; where a bar of light stimulates their excitatory simple cell but NOT its neighboring inhibitory simple cell

Question 25

What are orientation columns?

Answer 25

ORIENTATION COLUMNS: Neurons in primary visual cortex are highly organized according to stimulus specificity

• -Vertically, all neurons respond to same stimulus
• -Horizontally, neurons display a highly organized pattern of stimulus responsiveness

Question 26

When is the critical period for development of ocular dominance periods and why is it important?

Answer 26

Critical period = 0-6 years

• -Period during development in which the ocular dominance columns are developed
• -Can get permanent cortical blindness if one eye is sutured shut for too long or past the end of this time

Question 27

What is akinetopsia?

Answer 27

Motion blindness

A selective deficit in the detection of motion